Anemometers measure the flow of a fluid, such as air, passing over it. In electrical cabinets or boxes containing heat producing circuitry, it is sometimes necessary to provide a fan for cooling. It is conventional to provide instrumentation in the cabinet that monitors temperature and the flow of air. The most common compact anemometers are hot-wires and thermocouples.
The hot-wire type comprises a wire that has a known resistance-temperature characteristic. In one technique, the wire is heated by a current to achieve a target voltage across it and, as air flows over the wire to cool it, the current is controlled to maintain a constant voltage. The amount of current therefore corresponds to the air velocity once the anemometer is calibrated. Other techniques are also used, such as measuring the time it takes for the wire to cool after being pulsed by a current. Such wires are expensive, are fragile, require frequent cleaning, and require calibration.
A thermocouple is the coupling between two dissimilar metals, where the junction generates a temperature-dependent voltage. For use as an anemometer, as with the hot wire, the node may be heated by a current, and the voltage is measured. Like the hot wires, such thermocouples are expensive, fragile, require frequent cleaning (if exposed), and require calibration.
Hot wires and thermocouples are essentially mechanical devices since their characteristics are highly dependent on their mechanical construction. It would be desirable to create an anemometer using conventional integrated circuit fabrication techniques that is truly solid state, so that the anemometer is inexpensive, rugged, and does not require calibration.
U.S. Pat. No. 3,968,685 discloses an anemometer where a base-emitter voltage drop of a bipolar transistor is compared to a voltage drop across a reference (or compensation) diode. An adjustable current is provided to keep the base-emitter voltage drop equal to the diode voltage drop while the transistor is subjected to an air flow, and this current is equated to air velocity. The diode is use to compensate for ambient temperature changes. It is difficult to prevent the diode voltage drop from being affected by the air flow. Further, the design needs precise calibration to cancel out the voltage drops at ambient temperatures. Further, the transistor is thermally insulated by its plastic package, making its base-emitter junction slow to respond to changes in air velocity.
What is needed is a solid state anemometer that does not suffer from the drawbacks mentioned above and related drawbacks.